96 3500 hd rollback pto idle fluctuation
 

this truck has had a pto idle problem since i got it 6 years ago , it has 2 pto settings in the pcm and i have used both 1050 rpm and 1350 rpm but with the pto rpm swith on but pto not engaged it idles fine but as soon as i engage the pto the idle jumps up to 1350 and back to 700 , up down constantly which works the pto gears to death , some say inj pump what do you guys say?

I would have a look at the grounds at the back of the intake and the ground for the switch.

Since the idle control is ground activated.

i looked at those grounds and also put new grounds at the pcm plus i ran a new ground from the battery to the pto switch and it still surges

I would start with another ground also-the IP/PMD harness ground must be fastened to the IP and/or the passenger side battery ground directly on the battery ONLY.

If you have an intake mounted PMD cooler and the ground wire was fastened to it so the harness would reach,extend the wire to put it back on the IP or add another ground to the passenger side battery ground.

It would be best to also buy a 6' PMD extension harness and relocate the PMD cooler combo outside the engine bay behind the grill on the rad support or under/behind the frt bumper are good locations.

If that does not solve the issue I would check for trouble codes and ensure the CPS is working and is AC Delco ONLY-the tach signal to the ecm might be poor from an aftermarket CPS that are often bad right out of the box?

Then I would try another PMD and lastly the IP would be a suspect?